Day 10 - How do we decide where to dig?
I have been offsite much of the day today - this morning helping Carl with geophysics in a garden in Lenten Street and this afternoon pulling together content and photos for an article in the Alton Herald. However, I realised this morning that a question I get asked a lot by visitors to the site is: "How do you decide where to dig?"
Not so many years ago that answers would probably have been:
"We find a bit of buried wall and we dig along it until it stops." or
"We make an educated guess, perhaps based on something we see on the surface".
Anyone like me who grew up watching Tony Robinson and 'Time Team' for many years on Channel 4 will know that everything changed as a result of technology. These days we start before a dig by doing as much research as possible but also use several really valuable tools including:
Aerial photos. Sometimes you can ever see archaeological features such as cropmarks and parch marks on Google Earth. However, we tend to use other sources. Two I use very regularly are:
the Hampshire Data Portal, which has not only colour aerial photos of Hampshire but also false-colour infra-red as well, and can be found at http://hampshire.geodata.soton.ac.uk/#/
the Historic England archive of aerial photos, some of which date back to WW1. You can find out more at https://historicengland.org.uk/images-books/archive/collections/aerial-photos/ . This is a free service and if you complete a request giving a specific location they will look out all of their aerial images of that place. It takes several weeks and you then have to go in person to their offices in Swindon to view the prints - they are not online. If you find prints that are useful - and the best ones tend to be those taken by the RAF - you can pay 20p per print to copy them.
Lidar. Lidar is laser radar. Much but not all of Hampshire has been scanned using this tool by the Environment Agency, as part of their flood risk assessments. Aircraft fly over the countryside at about 1000ft and the scanner maps the terrain below, calculating the contours of the terrain. Its big advantage is that it can see through trees and undergrowth to map the ground below, so a great deal of archaeology has been revealed in forests and undergrowth. To get some idea, have a look at liderfinder.com.
Drones. I an ex-RAF and qualified some years ago as a commercial drone pilot and obtained a qualification called a PfCO (Permit for Commercial Operations) from the Civil Aviation Authority. I fly my drones for Liss Archaeology, partly for doing aerial photos of trenches (instead of having to hire scaffolding towers) and partly for doing surveys. I can cover a large site like Colemore in 20 minutes with the drone taking vertical photos every 5 seconds. I then use some superb but very expensive software (called 3D Zephyr Aerial) which not only creates a stitched-together photo called an 'orthomosaic' but also gives us a 3D virtual model of the site or dig area. Drones can measure the ground height to an accuracy of about 2cm and are excellent for picking up the outlines of villas and parch marks or cropmarks. This is an example of such a model:
Geophysics. Liss Archaeology's geophysics lead is Carl Raven and he has been trained by Historic England at Portsmouth and so is trusted to borrow from them some of the geophysics equipment that we cannot afford. We use two main techniques:
Resistivity. This is the machine you may have seen on Time Team which means walking up and down a 20 metre grid sticking a machine with spikes into the ground every half-metre. The machine measures how easily electricity flows through the moisture in the ground between a fixed probe and the two 'mobile probes' on the machine. Buried ditches and trenches under the ground tend to retain more moisture and allow electricity to flow more freely, whilst buried walls foundations obstruct the flow of electricity. We download our readings into a program called 'Snuffler' which produces a plot of buried features for us.
In the example above, you can see our dig area in Alton Public Gardens. I have chosen to produce it in colour. What you need to know to understand it in this case is that blue or purple denotes very moist/soft soil (as with the square purple feature on the left of the plot, which is the flower bed around the fountain just after watering). More important for us are the red and yellow areas, which indicate something very solid underground. You can see why our first test pits were on the circular feature at the centre (Victorian garden feature) and the diagonal from centre to top-left (buried pathway).
Magnetometry. This is a different geophysics technique that uses an extremely sensitive machine that picks up disturbances in the magnetic field caused by buried metallic objects or disturbance of the soil. It produces a plot similar to that for resistivity.
There is a third technique:
GPR. If you are really lucky, you also have GPR (ground-penetrating radar). This sends a radar signal into the ground that bounces back from any underground solid objects or voids. It is often used in building sites for finding buried pipes or electricity cables. We have been lucky in the past because another archaeology enthusiast who works mainly in Sussex, Dave Staveley, owns such a machine and he was kind enough to do some looking for us at Stroud last year. Dave is the author of the Snuffler software I referred to above and it is well worth reading his excellent blog at http://geophyswithsnuffler.blogspot.com/ if you want to learn more about what he does.
Finally, two photos taken while we were doing geophys in a garden in Lenten Street this morning: